Optical interposer for waveguides

ABSTRACT

A process for preparing a subassembly, the process comprising (a) defining the location of one or more grooves for receiving at least one polymer waveguide in a wafer, (b) etching the grooves into the wafer, each groove having sidewalls and a first facet at the terminal end perpendicular to the side walls, the first facet having a first angle relative to the top planar surface, (c) coating the first facet with a reflective material, and (d) disposing a fluid polymer waveguide precursor into each groove, and writing a core into the polymer material by directing at least one laser beam on the first facet by directing the laser beam into the top of the polymer material such that the beam reflects off of the first facet and down the interior of the polymer material to form the core in the polymer waveguide.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/017,668filed Jan. 31, 2011, which is a continuation-in-part of application Ser.No. 13/013,402, filed Jan. 25, 2011, which are hereby incorporated byreference.

FIELD OF INVENTION

The subject matter herein relates generally to fiber optic substrates,and more particularly, to an interposer with optical coupling andalignment features.

BACKGROUND OF INVENTION

Fiber optic components are used in a wide variety of applications. Theuse of optical fibers as a medium for transmission of digital data(including voice, internet and IP video data) is becoming increasinglymore common due to the high reliability and large bandwidth availablewith optical transmission systems. Fundamental to these systems areoptical subassemblies for transmitting and/or receiving optical signals.

Optical subassemblies typically comprise an interposer. As used herein,an interposer functions as a substrate for optical, opto-electrical, andelectrical components and provides interconnections to optically and/orelectrically interconnect the optical/opto-electrical/electricalcomponents. For example, a typical interposer may comprise a substrate,for example, silicon, having one or more grooves formed therein forsecuring an optical fiber. The conventional groove is formed in theshape of a “V” by wet etching the substrate to include two sidewallsthat retain the optical fiber along its length and an end face that isused as a mirror device. The conventional V-groove has a particularpitch α, which is the angle between the walls of the V-groove and a topor reference surface in which the V-groove was etched. Each of thesidewalls and the end face are typically formed at a precise angle of54.7 degrees from the reference surface due to the crystalline structureof silicon.

During operation, the end face of a conventional interposer V-groove ismetalized so that it may be used as a mirror to reflect light betweenthe optical/opto-electrical component and the optical fiber. Forexample, in the case of a transmitter, an opto-electrical light sourceemits a cone-shaped light beam onto the V-groove end face mirror. TheV-groove end face mirror reflects the light through an end of theoptical fiber retained in the V-groove. As discussed above, the surfaceof the V-groove end face is at an angle of precisely 54.7 degrees fromthe reference surface. As such, light is reflected off the groove endface mirror through the optical fiber at approximately −9.3 degrees fromthe reference surface and also from the longitudinal axis of the opticalfiber retained in the V-groove. Therefore, current devices utilizing theend face mirror of the groove to launch light through an end of theoptical fiber cause much of the light to be reflected away from the axisof the optical fiber resulting in non-optimal signal transmissionperformance.

Therefore, Applicants recognize that there is a need for an improvedoptical coupling between the optical/opto-electrical component and theoptical fiber or an optical planar waveguide. Additionally, Applicantsrecognize that this optical coupling should be achievable throughpassive alignment rather than active alignment to facilitate economicproduction of the subassembly. To this end, Applicants recently filed anew patent application (U.S. application Ser. No. 12/510,954,incorporated herein referenced) disclosing a multi-faceted fiber endface mirror for optical coupling. Specifically, the facets of the fiberend face mirror included a 54.7 degree facet to mechanically contact theend face of the V-groove to precisely position the optical fiber endface mirror in the V-groove along the longitudinal axis and under theemission aperture of the opto-electrical device. Additionally, anotherfacet was a 45 degree facet to facilitate optimal optical couplingbetween the optical axis of the fiber and the optical axis of theopto-electrical device. Additional facets were also disclosed forenhancing performance. Each of these facets would then be coated with ametal to act as a reflective mirror surface.

Although this development improved the optical performance andfacilitated passive alignment of the subassembly, it also requiredcoating the fiber end face on a number of different facets with ametallic/reflective coating. Applicants have identified an additionalneed to avoid the requirement for depositing a reflecting coating onfiber end faces as such a process tends to be difficult and expensiveand may be time prohibitive in a high volume application.

Therefore, a need exists for a simplified means for preparing an opticalassembly having good optical coupling to either an optical fiber or anoptical planar waveguide and performing this optical coupling by passivealignment. The present invention fulfills this need among others.

SUMMARY OF INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is notintended to identify key/critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

The present invention provides an interposer that functions as asubstrate for securing optical and/or opto-electrical components whileproviding a configurable reflective surface for optically coupling anoptical component with an optical fiber or with an optical planarwaveguide. Specifically, Applicants recognize that dry etchingtechniques can be used to configure a groove with a dry-etched facetthat is better suited to optically couple the optical component andfiber/planar waveguide than a traditional wet-etched facet of 54.7degrees. For example, in an embodiment in which the optical axis of thefiber/planar waveguide is essentially at a right angle to that of theoptical axis of the optical component, a 45 degree facet is dry-etchedinto the substrate. This facet is then coated with a reflective materialto act as a reflective surface/optical coupling mirror.

In one embodiment, the air gap between the reflective surface and thefiber/planar waveguide end face is reduced by configuring thefiber/planar waveguide end face with a corresponding mating facet havingessentially the same angle as the reflective facet at the terminal endof the groove. This way, when the fiber/planar waveguide is disposed inthe interposer groove, the facets of the substrate and fiber/planarwaveguide end face mechanically contact each other. Additionalcontacting or non-contacting facets may be added to the interposeroptical coupling mirror and fiber/planar waveguide end face to improveoptical coupling.

In one embodiment, the interposer has features that facilitates itsincorporation with planar waveguides. For example, because the edge ofthe interposer is precisely prepared, it provides a good interface withplanar waveguides. To this end, fiber stubs or discrete waveguides canbe disposed in the groove(s) between the optical component and the edgeto interface with the planar waveguide. Alternatively, the planarwaveguide may have discrete waveguide fingers defined at one end, witheach discrete waveguide finger being disposed in a groove of theinterposer. Still other configurations are possible using the interposerof the present invention.

In one embodiment, the interposer also has features that facilitatepassive alignment. For instance, as mentioned above, the groove has oneor more facets to receive the fiber/planar waveguide end face. Thisconfiguration enables the fiber/planar waveguide to be pushed forward inthe groove and when the corresponding facets contact, the fiber/planarwaveguide is positioned precisely on the interposer. Additionally, theinterposer may be provided with fiducials such as, for example, contactpads, visual markers or protrusions to register the optical component onits top planar surface relative to the positions of the fiber/planarwaveguide grooves.

In yet another embodiment, the interposer provides for electricalinterconnects between the optical component and related drive/receivecircuitry to interface the subassembly with a higher levelsource/receive component.

In light of the above, one aspect of the invention is an interposercomprising a configurable reflective surface for optically coupling thefiber/planar waveguide with an optical component. In one embodiment, theinterposer comprises: (a) a substrate having a top planar surface: (b)at least one groove defined in the top planar surface and extending froman edge of the substrate to a terminal end, the groove having side wallsand a first facet at the terminal end perpendicular to side walls, thefacet having a first angle relative to the top planar surface, the firstangle being about 45 degrees; (c) a reflective coating on the firstfacet; and (d) an optical conduit disposed in the groove and extendingbetween the first facet and the edge of the substrate.

Another aspect of the invention is a subassembly comprising theinterposer integrated with an optical component and an optical conduitsuch as a fiber/planar waveguide. In one embodiment, the subassemblycomprises: (a) an optical interposer comprising at least (i) a substratehaving a top planar surface; (ii) at least one groove defined in the topplanar surface and extending from an edge of the substrate to a terminalend, the groove having side walls and a first facet at the terminal endperpendicular to the side walls, the first facet having a first angle of45° relative to the top planar surface; and (iii) a reflective coatingon the first facet; (b) an optical fiber/planar waveguide disposed inthe groove having an optical axis and an end face; (c) an opticalcomponent having an optical axis perpendicular to the top planarsurface, the optic device being disposed on the top planar surface atthe terminal end such that its optical axis is accurately disposed overthe first facet such that the optical component is optically coupledwith high efficiency with the core of the optical fiber/planarwaveguide, and (d) a planar waveguide optically coupled to the opticalfiber/waveguide at the edge of the interposer.

Another aspect of the invention is a process of preparing theinterposer. In one embodiment, the process comprises: (a) defining thelocation of one or more grooves for receiving fibers/planar waveguidesfor multiple interposers on a top surface of common wafer; (b) definingthe location of patterns of contact pads on the top surface, eachpattern configured to receive an optical component; (c) defining thelocation of electrical traces interconnecting the contact pads; (d)etching the grooves into the wafer, each groove having sidewalls and afirst facet at the terminal end perpendicular to the side walls, thefirst facet having a first angle relative to the top planar surface; (e)coating the first facet with a reflective material; (f) depositing theelectrical traces; and (g) depositing the contact pads.

Yet another aspect of the invention is a process of preparing asubassembly in which the interposer is coupled to a planar waveguide. Inone embodiment, the process comprises: (a) defining the location of oneor more grooves for receiving optical conduits in a wafer; (b) etchingthe grooves into the wafer, each groove having sidewalls and a firstfacet at the terminal end perpendicular to the side walls, the firstfacet having a first angle relative to the top planar surface; (c)coating the first facet with a reflective material; (d) disposing anoptical conduit in each groove to optically couple the first facet witha planar waveguide. In one embodiment, a planar waveguide is disposed atthe edge of the interposer and is optically coupled to the opticalconduit. In another embodiment, the optical conduit is integral with theplanar waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a substrate having U-groove of the presentinvention.

FIG. 2 shows an image of a cross section of the terminal end of aU-groove of the present invention.

FIG. 3 shows one embodiment of the present invention showing multiplestepped V-grooves and contact pad patterns for receiving anopto-electrical device and related electrical circuitry.

FIG. 4 shows a completed opto-electrical interposer showing theopto-electrical device and related electrical circuitry connected to anarray of optical fibers/planar waveguides.

FIG. 5 shows a side schematic view of a fiber having a profiled end facefor seating in the interposer of the present invention.

FIG. 6 shows the interposer of FIG. 1 with fibers disposed in each ofthe grooves.

FIG. 7 shows a cross sectional view of the interposer shown in FIG. 4with a planar waveguide disposed in the groove.

FIG. 8 shows a cross section of an interposer along a groove in whichthe terminal end of the groove has an additional facet.

FIGS. 9 and 10 show the substrate of the interposer of FIG. 8 with anon-profiled fiber disposed therein.

FIG. 11 shows another embodiment of the interposer of the presentinvention in which the groove has an additional facet similar to that ofFIG. 8.

FIG. 12 shows an embodiment of the interposer coupled to a planarwaveguide.

FIG. 13 shows a portion of the substrate of one embodiment of theinterposer having a visual fiducial for disposing the optical conduit inthe groove.

FIG. 14 shows an embodiment of the interposer in which the groove isfilled with a polymer waveguide.

FIG. 15 shows a schematic of the interposer of FIG. 14 in whichcollimated laser beam is directed onto the first facet and is turneddown the length of the polymer waveguide to write a core into thewaveguide.

FIG. 16 shows the core produced in the polymer waveguide from thewriting process illustrated in FIG. 15.

FIG. 17 shows a pair of transmitting/receiving interposers arranged on awafer such that the optical conduit in the groove connections theoptical components (not shown) of the interposer.

FIG. 18 shows the interposer incorporated into a planar waveguide.

FIG. 19 shows an end of ribbon waveguide in which the discrete waveguidefingers have been defined.

FIG. 20 shows a side view of the interposer of the present invention, inwhich the grooves are suitable for receiving the discrete waveguidefingers shown in FIG. 19.

DETAILED DESCRIPTION

Referring to FIG. 1, one embodiment of an optical interposer 100 of thepresent invention is shown. The optical interposer 100 comprises asubstrate 101 having a top planar surface 102 and at least one groove103 defined in the top planar surface, extending from an edge 106 of thesubstrate to a terminal end 107. The groove 103 has side walls 104 and afirst facet 105 at the terminal end, which is essentially perpendicularto side walls. The first facet is configured at a first angle α relativeto the top planar surface. A reflective coating (not shown) is depositedon the first facet 105.

Referring to FIG. 12, another embodiment of a subassembly 1200comprising the interposer 1250 integrated with a waveguide 1260 isshown. Like the interposer 100 described above, the interposer 1250comprising at least a substrate 1201 having a top planar surface 1202and at least one groove 1203. The groove is defined in the top planarsurface and extends from an edge 1206 of the substrate to a terminal end(not shown). Like the groove 103 described above, groove 1203 has sidewalls and a first facet at the terminal end perpendicular to the sidewalls, and the first facet has a first angle relative to the planarsurface. A reflective coating is disposed on the first facet. In thegroove 1203 is disposed an optical conduit 1261, which extends to theedge 1206. The optical conduit has an optical axis and an end face withat least a first mating facet having essentially the first anglerelative to its optical axis such that the first mating facet and thefirst facet contact each other. The subassembly 1200 also comprises anoptical component 1220 having an optical axis (not shown) perpendicularto the top planar surface. The optic component is disposed on the topplanar surface 1202 at the terminal end such that its optical axis isdisposed over the first facet such that the optical component isoptically coupled with the core of the optical conduit. Disposed at theedge 1206 is a waveguide 1260 optically coupled to the optical conduit1261 at the edge.

Each of these elements and alternative embodiments are described ingreater detail below.

A primary function of the interposer is to provide a substrate orbackbone to support and secure an optical conduit, optical component(s)and supporting electrical circuitry. As used herein, an optical conduitrefers to any know medium for facilitating the propagation of opticalsignal in a certain direction. Common optical conduits include, forexample, optical fibers and planar waveguides. To support the opticalconduit, the substrate should comprise a rigid material that can beetched or machined to define the grooves and is thermally stable,suitable for being heated to temperatures typical in solder reflowapplications. Examples of suitable materials include elemental materialshaving a crystalline form, polymeric material, glass, ceramics (i.e.,oxides, nitrides, carbides, borides and silicides of metals orsemi-metals and combinations thereof), quartz, and metals.

The optical conduit, which may be an optical fiber or planar waveguide,is disposed in each of the grooves 103. For example, referring to FIG.6, an interposer 600 is shown having a plurality of grooves 601 definedin the top surface of the substrate 602. A fiber 603 is disposed in eachgroove such that the end face 606 of the fiber is in physical contactwith a first facet 605 of the terminal end of the groove 601. (Alsoshown in FIG. 6 are contact pads 604 for electrically connecting andaligning the optical component as described below.) Referring to FIG. 5,the optical fiber 500 has an optical axis 501, a core 502, and an endface 503 with at least a first mating facet 504 at essentially the firstangle α, relative to the optical axis 501 such that the first matingfacet and the first facet of the terminal end make physical contact witheach other when the fiber is disposed in the groove as shown in FIG. 6.

In the embodiment in which the interposer 1250 of the present inventionis combined with a waveguide 1260 as shown in FIG. 12, the opticalconduit 1261 functions to optically couple the first facet to thewaveguide 1260, and may have a number of different embodiments. In oneembodiment, the optical conduit 1261 is a fiber stub in which one end isconfigured as described above to interface with the first facet, whilethe other end is configured to interface with the waveguide 1260. Tothis end the fiber stub may have a variety of different core diameters(e.g. 35, 45, 50, or 62.5 μm) and it may be a gradated index or a stepindex.

Alternatively, the optical conduit 1261 may be a polymer waveguide.Different embodiments of the polymer waveguides are possible in thepresent invention. For example, in one embodiment, the polymer waveguide1600 comprises a core 1601 defined in its interior to contain thepropagating light as shown in FIG. 16. The core case be any size (e.g.35, 45, 50, or 62.5 μm). Alternatively, rather than having the coredefined in the polymer waveguide, the polymer waveguide may be just thecore—i.e. it may comprise only core material. To this end, the groovesmay be etched to correspond to the core size (e.g. 35, 45, 50, or 62.5μm). To ensure light is reflected by the side walls and the bottom ofthe groove (and not absorbed by the substrate), the walls and bottom maybe coated with a reflective material. The difference between therefractive indices of the polymer waveguide and air prevents the laserlight from escaping through the top of the waveguide.

In yet another embodiment, the waveguide 1260 is integrally-formed withthe optical conduit 1261. For example, referring to FIG. 19, one end ofthe waveguide 1260, which, in this embodiment, is a ribbon waveguide1260 a, may be modified such that individual waveguides are presented indiscrete waveguide fingers 1261 a. Each finger 1261 a may be insertedinto a corresponding groove 2003 of the interposer 2050 of thesubassembly 2000 shown in FIG. 20.

In one embodiment, the parallel side walls of the groove hold the fiberin place. (Although single fiber applications are shown and describedherein, it should be appreciated that the invention is not limited tosingle fiber applications and may be applied to arrays of fibers andribbon fiber and to planar waveguide arrays and ribbons as well.) Thesidewalls may be the walls of a traditional V-groove, or they may beperpendicular to the top planar surface such that they form more of aU-groove. As mentioned above, wet etching may be used to form V-grooves,while dry etching as discussed below can be used to form any side wallconfiguration because the etching process is not dependent on thecrystalline structure of the substrate.

In one embodiment, the sidewalls are configured to secure a strippedfiber. For example, referring to FIG. 3, the interposer 300 is shownwith stepped V-grooves 303. Like interposer 100 in FIG. 1, interposer300 comprises a substrate 301 having a top planar surface 302 and aplurality of stepped grooves 303 defined therein. Each groove comprisessidewalls 304 and a terminal end 307 perpendicular to the sidewalls 304.A terminal end comprises a first facet 305 as described above. Unlikethe embodiment shown in FIG. 1, however, the grooves are V-grooves 303,which are stepped and have a wide portion 309 and a narrow portion 308.The wide portion 309 is configured to receive both the fiber and itsbuffer coating, while the narrow section 308 is configured to receivejust the bare fiber. As is known in the art, the buffer coating can bestripped from the fiber, leaving just the core and cladding, which isreferred to as “bare fiber.”

The optical conduit may be secured to the groove in a variety of knownways. For example, a fiber may be metalized and soldered in place or itmay be glued in place. In one embodiment, a UV-cured, opticallytransparent adhesive is used to secure the fiber/planar waveguide in thegroove. Such an approach may be preferable to reduce Fresnel losses, asany gaps between the optical component, the terminal end of the grooveand the end face of the fiber/planar waveguide would be filled with theoptically transparent adhesive.

The optical component may be any known component that is opticallycoupled to a fiber/planar waveguide. The optical component may be forexample (a) a passive component, which does not convert optical energyto another form and which does not change state (e.g., fiber, lens,add/drop filters, arrayed wave guide gratings (AWGs), GRIN lens,splitters/couplers, planar waveguides, or attenuators); (b) an activedevice which converts between optical energy and electrical energy(e.g., lasers, such as vertical cavity surface emitting laser (VCSEL),double channel, planar buried heterostructure (DC-PBH), buried crescent(BC), distributed feedback (DFB), distributed bragg reflector (DBR);light-emitting diodes (LEDs), such as surface emitting LED (SLED), edgeemitting LED (ELED), super luminescent diode (SLD); and photodiodes,such as P Intrinsic N (PIN) and avalanche photodiode (APD)); or (c) ahybrid device which does not convert optical energy to another form butwhich changes state in response to a control signal (e.g., switches,modulators, attenuators, and tunable filters). It should also beunderstood that the optical component may be a single discrete device orit may be assembled or integrated as an array of devices.

The optical component has at least one optical axis along which thelight propagates to/from the optical component. Because the opticalcomponent is disposed over the fiber/planar waveguide and opticallycoupled thereto by virtue of a reflective surface defined in theinterposer, generally, although not necessarily, the optical axis isessentially perpendicular to the planar surface. It should be understoodthat the optical component is not limited to a single optical axis. Forexample, in the embodiment shown in FIG. 4, the optical component iseither a VCSEL array or a PIN array, in which the optical component hasmultiple optical axes.

In one embodiment, the interposer functions not only to support afiber/planar waveguide and optical component, but also to opticallycouple them at high efficiency. To this end, a significant feature ofthe present invention is the groove 103 with first facet 105,specifically configured to facilitate optical coupling between theoptical component and the fiber/planar waveguide. Applicants recognizethat dry etching techniques can be used to specifically configure theterminal end of the grooves to provide the optimal first angle α. Dryetching refers to the removal of material typically using a maskedpattern by exposing the material to a bombardment of ions (usually aplasma of reactive gases such as fluorocarbons, oxygen, chlorine, borontrichloride; sometimes with addition of nitrogen, argon, helium andother gases) that dislodge portions of the material from the exposedsurface. Unlike typical wet etching, dry etching typically etchesdirectionally or anisotropically, and thus is not dependent on thecrystalline structure of the substrate.

Because dry etching is not limited or controlled by the crystallinestructure of the underlying substrate (unlike traditional wet etching ofsilicon, which will generally result in a V-groove having a wall slopeof 54.7 degrees as mentioned above), dry etching can be used to producewall slopes of any desired angle in a wide variety of substratematerials. Accordingly, in one embodiment, the first facet 105 isconfigured with an optimum angle to result in efficient optical couplingbetween the optical component and the core of the fiber/planarwaveguide. Generally, although not necessarily, this angle will be abouta 45 degree angle if the optical axis of the fiber/planar waveguide isat a right angle to optical axis of the optical component.

At least the first facet is treated to make it reflective. For example,it may be coated with a metal or other reflective material as is know inthe art. Suitable reflective materials include, for example, gold,silver, aluminum and dielectrics. The materials may be deposited on thefacets using known techniques, including, evaporation, sputtering andvapor deposition.

Additional facets may be added to the groove to improve optical couplingand/or to improve passive alignment. For example, referring to FIG. 2,an image of the cross section of a U-groove is shown. The substrate 201defines a terminal end having a first facet 202 at essentially a 45degree angle with respect to the planar top surface 204. This image alsoshows a second facet 203 at a steeper (i.e. greater) angle with respectto the top planar surface. Although not necessary, a second facet 203may be preferable under certain circumstances to provide for enhancedlocating of the fiber/planar waveguide in the groove. Specifically,because the angle of second facet 203 is greater than that of firstfacet 202, it will tend to function as a mechanical stop to preventforward axial movement of the fiber/planar waveguide more effectivelythan the first facet because of a reduced upward wedging force betweenit and the corresponding mating facet of the fiber/planar waveguide.

Other facets may be added to the terminal end of the groove to furtherenhance optical coupling. For example, as described in U.S. applicationSer. No. 12/510,954 with respect to reflective facets on the fiber endface, facets that gradually angle away from the first facet (forexample, in 5° increments) tend to enhance optical coupling by focusingdivergent light. Still other embodiments of the groove and terminal endfacets are possible in light of this disclosure.

Referring to FIG. 8, another embodiment of an interposer 800 is shown,in which a U-groove 802 is defined in a substrate 801 along with aterminal end that is defined by multiple angled faces. In thisembodiment, the terminal end is etched in the substrate before theU-groove. The terminal end of this embodiment is masked as a square andthen four sides walls are etched from each side of the square (see FIG.9 for a top view). Each side is etched at a 45° angle relative to thetop surface of the substrate 801. One of the sides is the first facet803 as shown. After the terminal end is etched, the U-groove is etched.In this embodiment, the U-groove is deeper than the terminal end suchthat a second facet 804 is defined.

Although a square is used to define the terminal end in the embodimentof FIG. 8, it should be understood that other shapes are possible,including polygons with more than four sides. Polygons with multiplesides, particularly a concentration of sides adjacent or near the firstfacet may be desirable to provide additional facets to focus light asdescribed above. That is, facets that angle gradually away from thefirst facet tend to improve optical coupling by focusing the light moreeffectively.

Referring to FIG. 11, another embodiment of the U-groove shown in FIG. 1is shown. Specifically, interposer 1100 shown in FIG. 11 comprises aU-groove 1101 having a terminal end 1102 with a first facet 1103 at a45° angle with respect to the top surface 1104 as disclosed in theprevious embodiments discussed above. However, like the embodiment shownin FIG. 8, this embodiment defines a second facet 1105, which, asdiscussed with respect to the second facet shown in FIG. 8, may be usedas a stop for the fiber/planar waveguide to enhance its axiallypositioning/alignment in the groove 1101.

To enhance the effectiveness of the optical coupling, it may bepreferable in certain applications to reduce the air gap between theoptical component, the facets of the groove and the fiber/planarwaveguide end face. Accordingly, in one embodiment, the fiber/planarwaveguide end face is configured to have the same profile as that of theterminal end of the groove. To this end, the fiber/planar waveguide endface has at least a first mating facet configured to contact the firstfacet when the fiber/planar waveguide is dispose in the groove andpushed against the terminal end of the groove. Specifically, referringto FIG. 5, a schematic of a cross section of a fiber of the presentinvention is shown. The fiber 500 comprises an optical axis 501 and acore 502. A first mating facet 503 is defined on the end face of thefiber. At least a portion of the first mating facet 503 is defined bythe core 502 of the fiber.

As mentioned above, it may be desirable in certain applications tofurther profile the end face of the fiber for additional opticalperformance and/or passive alignment. For example, fiber 500 comprises asecond mating facet 504, which is at an angle greater than that of thefirst mating facet 503. Such a configuration may be preferable forpositioning the fiber passively as any forward motion on the fibertoward the terminal end of the groove will tend to result in less upwardforce because of wedging action between the second facet of the grooveand the second mating facet of the fiber 504. Additionally, it may bepreferable to further enhance the optical end face with a third matingfacet 505, which is at a greater angle than that of first mating facet503. Additionally, side facets may be added to the fiber end face oneither side of the first mating facet to enhance optical coupling. (see,for example, U.S. application Ser. No. 12/510,954, incorporated hereinby reference.) In one embodiment, to the extent the fiber end face hasmultiple facets, the terminal end of the groove is profiled to havecorresponding facets such that the end face of the fiber is received bythe terminal end of the groove with minimal air gaps.

Although physical contact between the first mating face of thefiber/planar waveguide and the first facet of the terminal end may bedesirable, it is not necessary, and, in certain applications, a spacemay be desired to facilitate manufacturability. For example, referringto FIGS. 9 and 10, an alternative embodiment of the interposer of thepresent invention is shown. Specifically, interposer 900 in thisembodiment comprises a fiber with a non-profiled end face 902. Morespecifically, referring to FIG. 10, the fiber 901 is disposed in thegroove 802 (it should be noted that the interposer 900 has the samesubstrate and groove as disclosed in FIG. 8) such that the non-profiledfiber end face 902 abuts against the second facet 804 to axially alignthe fiber in the interposer 900. In this way, the core of the fiber 901is optically coupled to the optical component disposed above theterminal end (not shown for simplicity) via the first facet 803, whichis coated with a metallic surface as described above. This embodimentoffers certain advantages over the profiled fiber end disclosed in FIG.5. In particular, end faces that are normal to the optical axis arerelatively easy to manufacture and can be prepared using standardmechanical cleaving or laser cleaving techniques.

In addition to improved manufacturability due to the non-profiled fiberend face 902 and good axial alignment due to the second facet 804abutting the fiber end face 902, the embodiment of FIGS. 9 and 10, alsohas the benefit of a relatively short optical path between the fibercore 903 and the first facet 803 because the lower portion of the firstfacet is truncated by groove 802. That is, by etching away the lowerpart of the first facet 803, the core of the fiber can be closer to thereflective surface, which reduces light diffusion and thus improvesoptical coupling. Therefore, the configuration of the groove andterminal end shown in FIGS. 8, 9 and 10, not only produces a secondfacet for axial registration of the fiber in the groove, but alsoshortens the first facet allowing the core of the fiber to get closer tothe reflective surface.

Nevertheless, it should be understood that because there is an air gapbetween the non-profiled end face 902 and the first facet 803, opticalperformance may still be comprised to some degree. However, this gap maybe filled with an optically transparent gel/adhesive or similarsubstance to improve or enhance the optical coupling between the fiber901 and the optical component (not shown) disposed above the terminalend of the groove 802.

Considering now the disposition of a polymer waveguide in the groove,several process alternatives are available. For example, in theembodiment of FIG. 16 in which the core is defined in the interior ofthe waveguide, a number of techniques may be used. In one embodiment,the process comprises pouring a fluid polymer waveguide precursor intoeach groove 1403 and curing the precursor to form the polymer waveguide1401 as shown in FIG. 14. In this embodiment, the polymer material usedis the type that forms a core material upon contact with one or morelaser beam(s) of a certain intensity. (Such polymer materials areknown.) Typically, converging laser beams are used. Next, as shown inFIG. 15, one or more laser beams are focused on the first facet 1501such that the initial beam(s) 1502 are turned, and then the beams 1503propagate down the waveguide 1401, forming the core 1601 (see FIG. 16)as they go. As shown in FIG. 16, this results in an first core 1602starting at the top surface of the waveguide 1600 and a second core 1601are essentially a right angle to the first core 1602 which travelsaxially down the waveguide 1600.

In another embodiment, the core is defined directly in the waveguide. Tothis end, the groove is filled with polymer waveguide precursor to apoint corresponding to the bottom of the core. Then another layer ofpolymer waveguide precursor is applied, but this layer is formulatedspecifically to turn into core material upon contact with a laser beamof sufficient energy. Next, one or more lasers are used to write a coredown the middle of the waveguide, thus turning that polymer materialinto a core. The specially-formulated material on either side of thecore, which did not come in contact with the laser beam, is then washedaway, and a top layer of polymer material is applied to encapsulate thecore.

In another embodiment, discrete waveguide fingers 1261 a are formed bystripping away waveguide material as shown in FIG. 19. This fingers 1261a may then be inserted into the grooves of the interposer as discussedabove.

Still other ways of disposing a waveguide in the grooves of theinterposer will be obvious to one of skill in the art in light of thisdisclosure.

The interposer of the present invention also comprises features forpassively aligning the fiber/planar waveguide and the optical component.One of the primary technical challenges associated with the manufactureof optical assemblies, especially systems offering higher levels ofintegration, is component optical alignment. This is especiallyapplicable in free-space, interconnect optical systems where discreteoptical components, such as active devices (e.g., semiconductor lasers),passive devices (e.g., filters), and/or MOEMS (micro-opticalelectromechanical systems) (e.g., tunable filters and switches) areintegrated on a common mounting system to exacting tolerances, typicallyin the sub-ten micrometer down to sub-micrometer range.

There are generally two alignment approaches for aligning opticalcomponents—active and passive. In passive alignment, registration oralignment features are typically fabricated directly on the componentsas well as on the platform to which the components are to be mounted.The components are then positioned on the platform using the alignmentfeatures and affixed in place. In active alignment, the opticalcomponents are placed on the platform, but before being affixed thereto,an optical signal is transmitted through the components while they aremanipulated to provide optimum optical performance. Once optimumperformance is achieved, the components are affixed to the platform.Although active alignment tends to be more precise than passivealignment, passive alignment facilitates high-speed, high-volumeautomated manufacturing and, thus, is preferred. It tends to beexceedingly difficult, however, to optically align in all three axesusing passive alignment, especially if exceptionally good alignment isrequired. Nevertheless, a significant reduction in manufacturing timeand costs can be realized if passive alignment can be used to achieveacceptable alignment along two axes or even one so that active alignmentis only necessary for the remaining axes or for fine tuning.

The interposer of the present invention may have a number of features tofacilitate passive alignment of the fiber/planar waveguide and/oroptical component. For example, as already mentioned above, tofacilitate passive alignment of the fiber/planar waveguide in theinterposer, the terminal end of the groove is profiled to receive theend face of the fiber/planar waveguide. This allows the fiber/planarwaveguide to be pushed into the interposer until its end stubs againstthe terminal end of the groove. Because the terminal end is profiled toreceive the end face of the fiber/planar waveguide, the fiber/planarwaveguide end face seats against the terminal end along one or moremating facets, thus ensuring a precise positioning. To further enhancethis alignment, in one embodiment, at least a second facet of theterminal end has a relative steep angle with respect to the top planarsurface to reduces the upward wedging force on the fiber/planarwaveguide as it is pushed forward into the terminal end of the groove.To this end, the second facet is greater than 45 degrees as shown inFIG. 2. Another approach for limiting the upward movement of thefiber/planar waveguide caused by the wedging action of the first facetagainst the mating first facet of the groove is to position the firstoptical component immediately over the terminal end of the groove suchthat the clearance between the top of the fiber/planar waveguide and thebottom of the optical component is tight, thus limiting the amount ofupward movement, and hence axial movement, of the fiber/planar waveguidein the groove.

In one embodiment, the interposer also has fiducials to facilitatepassive alignment of the optical component such that each of its opticalaxes are aligned with its respective first facet of the groove.Fiducials may be any structure or marking which provides for the passivealignment of the optical component. A variety of fiducials may be used.In one embodiment, a pattern of contact pads are used that passivelyalign the optical component during a reflow operation. Specifically, theoptical component is provided with a certain pattern of contact pads onits bottom, the interposer has the same pattern on its top planarsurface. The optical component is then placed on the pads in roughalignment using known pick and place technology. Alignment between theinterposer and optical component is then achieved when the assembly isreflowed such that the surface tension of the contact pads causes thepatterns of the optical component to align over the pattern on theinterposer, thereby precisely positioning the optical component relativeto the grooves of the interposer. Such a mechanism is well known anddisclosed, for example, in U.S. Pat. No. 7,511,258, incorporated hereinby reference.

In another embodiment, rather than or in addition to contact pads, otherfiducials on the interposer are used to facilitate passive alignment.For example, the fiducials may be physical structures protruding fromthe planar surface that provide a register surface against which theedge of the optical component may contact to be positioned correctly onthe interposer. Alternatively, the fiducials may be markings to enablevisual alignment of the optical component on the interposer using acommercially-available, ultra-high precision die bonding machine, suchas, for example, a Suss MicroTec machine (See, e.g., U.S. Pat. No.7,511,258).

Referring to FIG. 13, yet another use of fiducials is shown. In thisembodiment, a visual fiducial 1301 is applied during photolithography aprecise distance from the first facet 1302. This way, if the first facet1302 is obscured by the optical component or any other component, thepick and place assembly equipment can position the optical conduit inthe groove based on the visual fiducial.

Additionally, a combination of fiducials and contact pads may be used.For example, the pads may be used to pull the optical component intocontact with the raised fiducials of the interposer. Still otheralignment techniques will be apparent to one of skill in the art inlight of this disclosure.

The interposer may also have circuitry (electrical/optical) forproviding the necessary interconnections for supporting the opticalcomponent. For example, referring to FIGS. 4 and 7, an interposer 400 isshown comprising a substrate 401 having a top planar surface 402 and aseries of grooves 403. The optical component 420, in this case, an arrayof VCSELs or PIN photodiodes, is disposed over the terminal ends of thegrooves and aligned with pads 310 (as shown in FIG. 3) and opticallycoupled with the fibers 404. The required integrated circuit 430 for theoptical component 420 is disposed proximate the optical component 420and is also aligned with pads 311 (see FIG. 3). The pads 310 and 311 areinterconnected with electrical traces (not shown for simplicity).Additionally, other traces (again, not shown for simplicity)electrically connect the integrated circuit 430 to through substrate 401vias 440 along the perimeter of the interposer 400. The viaselectrically interface the interposer with the higher level flex circuitor printed circuit board 702 through contact pads 701. This is a knowntechnique.

Therefore, the interposer of the present invention may have one or morefeatures for optically coupling an optical component to a fiber/planarwaveguide, features for providing passive alignment of the fiber/planarwaveguide and/or optical component, and electrical/opticalinterconnections for interconnecting the optical component with requiredcircuitry and for interfacing the interposer with the higher level flexcircuit or printed circuit board.

The interposer of the present invention also lends itself to economicaland highly repeatable manufacturing. In particular, most if not all ofthe critical alignment relationships may be defined on the wafer scale,often in just a few, or even a single, photolithography step.Specifically, the location of the grooves for holding the fiber/planarwaveguide and the contact pads for electrically connecting and providingpassive alignment of the optical components may be defined in a singlemasking step. Additionally, in one embodiment, the optical/electricalinterconnections among the various components may be defined in a singlemasking step. For example, the various traces interconnecting the padsfor the optical component and the pads for the electrical drivercircuitry, and the traces between the driver circuitry and the throughsubstrate vias may be defined in a single masking step. In oneembodiment, even the edges of the interposers are defined in the samemasking step. In other words, each edge 320 of the interposer as shownin FIG. 3 is one half of a groove etched in the wafer. The wafer issimply parted at the bottom of each groove to form interposers withprecisely controlled edges. This way, the distance from the edge 320 ofthe interposer to critical features such as the grooves 303 may beprecisely controlled, often in a single step, thereby eliminatingtolerance build up and simplifying assembly manufacturing with theinterposer by use of these precisely controlled edges.

The etching may also be performed on wafer-scale. In one embodiment, thegrooves, terminal end facets, and the edges of the interposer are alldefined and etched at the wafer-scale. Further economies may be realizedby etching these features in the same photolithographic procedure.Although a single etching procedure may be used, in certaincircumstances, two or more etching procedures may be beneficial. Thatis, the facets of the interposer require dry etching to achieve thedesired slope, however, the sidewalls of the interposer and the edges ofthe grooves may be etched using either wet or dry etching techniques.Therefore, if the dry etching is not as economical as wet etching (e.g.,it takes longer and/or is more expensive), then it may be preferable toetch the terminal end facets using dry etching and the interposer andgroove sidewalls/edges using wet etching.

In one embodiment, the interposers are fabricated in pairs on a wafer.Specifically, referring to FIG. 17, a first interposer 1701 is atransmitting interposer, meaning it has a transmitting optical component(not shown) disposed on pads 1703, and a second interposer 1702 is areceiving interposer having a receiving optical component (not shown) onpads 1704. By preparing interposers in transmitting/receiving pairs, thepair may be tested before being integrated with a waveguide or othercomponents of a subassembly. Specifically, before the individualinterposers are diced along the indicated grooves 1706, they may befitted with their optical components and related circuitry andenergized. This way light would be generated at the first interposer1701, transmitted down waveguide 1705 and into the second interposer1702. Any defects in the circuitry and alignment of the components wouldbe evident at this point, before incorporation of the interposer intothe subassembly, thus saving time and material.

The incorporation of the waveguide with the interposer may be performedin various ways. For example, as shown in FIG. 12, the interposer 1250may be integrated with the waveguide 1260 via a common carrier 1270,such as a flex circuit. Alternatively, the interposer may be integrateddirectly into a ribbon waveguide. For example, referring to FIG. 18, awaveguide 1860 is shown with a notch 1808 cut out of one end of it. Thenotch has a notch edge 1807 which is U-shaped and configured toprecisely receive the edge 1806 of the interposer 1850. (As describedabove, the edge of the interposer of the present invention can becontrolled with a high degree of precision.) In this way, the waveguide1860 receives the interposer 1850 and holds it precisely with respect tothe waveguide cores 1809. In one embodiment, the optical conduits 1861in the grooves 1803 are aligned with the cores 1809 in the ribbonwaveguide 1860. In other embodiment, the individual waveguide are cutaway from the surrounding material of the ribbon waveguide (see, e.g.,FIG. 19), such that discrete waveguide fingers are inserted into thegrooves 1803 as described with respect to FIG. 20.

It should be apparent from the above description that the interposerassembly of the present invention provides for significant advantagesover conventional electro-optic module configurations such as lower costand simplicity in manufacturing and enhanced versatility with respect tothe type of mating components with which it can effect opticallycoupling. Still other advantages of the interposer assembly areanticipated.

What is claimed is:
 1. A process for preparing a subassembly, saidprocess comprising: (a) defining the location of one or more grooves forreceiving at least one polymer waveguide in a wafer; (b) etching saidgrooves into said wafer, each groove having sidewalls and a first facetat said terminal end perpendicular to said side walls, said first facethaving a first angle relative to said top planar surface; (c) coatingsaid first facet with a reflective material; and (d) disposing a fluidpolymer waveguide precursor into each groove, and writing a core intosaid polymer material by directing at least one laser beam on said firstfacet by directing said laser beam into the top of said polymer materialsuch that said beam reflects off of said first facet and down theinterior of said polymer material to form said core in said polymerwaveguide.
 2. The process of claim 1, wherein step (d) comprises using avisual fiducial to dispose said optical conduit in said groove.
 3. Theprocess of claim 1, further comprising: filling said groove with apolymer waveguide cladding precursor to a point corresponding to thebottom of a core; applying a layer of polymer waveguide core precursor;writing a core down the middle of the waveguide with one or more laserbeam(s), thus turning that polymer core precursor into a core; removingsaid polymer waveguide core precursor on either side of the core thatdid not come in contact with said laser beam(s); and applying a toplayer of polymer clad material to encapsulate the core.
 4. The processof claim 1, wherein, in step (a), said grooves are defined for multipleinterposers on a common wafer.
 5. The process of claim 4, wherein saidmultiple interposers are arranged in pairs, a transmitting interposerand a receiving interposer, and share common grooves prior to beingdiced.
 6. The process of claim 5, further comprising placing atransmitting optical component and associated circuitry on eachtransmitter interposer proximate said contact pads defined in saidwafer; placing a receiver optical component and associated circuitry oneach receiver interposer proximate said contact pads defined in saidwafer; reflowing said contact pads such that each optical component ispulled into alignment via the surface tension of said contact pads. 7.The process of claim 6, further comprising: testing each pair todetermine if transmitting and receiving optical components arefunctioning correctly before dicing said wafer.
 8. An optical interposercomprising: a substrate having a top planar surface: at least one groovedefined in said top planar surface and extending from an edge of saidsubstrate to a terminal end, said groove having side walls and a firstfacet at said terminal end perpendicular to side walls, said facethaving a first angle relative to said top planar surface, said firstangle being about 45 degrees; a reflective coating on said first facet;an optical conduit disposed in said groove and extending to said edge,said optical conduit having an optical axis and an end face with atleast a first mating facet having essentially said first angle relativeto its optical axis such that said first mating facet and said firstfacet contact each other.
 9. The interposer of claim 8, wherein saidside walls and bottom of said groove are coated with a reflectivematerial, said wherein said groove is sized to correspond to a core ofan optical conduit and is filled only with polymer core material. 10.The optical interposer of claim 8, further comprising fiducials forpassively aligning an optical component relative to said first facet.11. The optical interposer of claim 8, further comprising at least onevisual fiducial proximate said groove for disposing said optical conduitin said groove.
 12. The interposer of claim 8, wherein said substrate isa portion of a wafer comprising additional interposers.
 13. Asubassembly comprising: an optical interposer comprising at least asubstrate having a top planar surface: at least one groove defined insaid top planar surface and extending from an edge of said substrate toa terminal end, said groove having side walls and a first facet at saidterminal end perpendicular to said side walls, said first facet having afirst angle relative to said planar surface; a reflective coating onsaid first facet; an optical conduit disposed in said groove andextending to said edge, said optical conduit having an optical axis andan end face with at least a first mating facet having essentially saidfirst angle relative to its optical axis such that said first matingfacet and said first facet contact each other; and an optical componenthaving an optical axis perpendicular to said top planar surface, saidoptic component being disposed on said top planar surface at saidterminal end such that its optical axis is disposed over said firstfacet such that said optical component is optically coupled with saidcore of said optical conduit; and a waveguide optically coupled to saidoptical conduit at said edge.
 14. The subassembly of claim 13, whereinsaid first angle is about 45 degrees.
 15. The subassembly of claim 13,wherein said optical conduit is a polymer waveguide having a coredefined in its interior.
 16. The subassembly of claim 15, wherein saidpolymer waveguide and said waveguide are integrally formed.
 17. Thesubassembly of claim 16, wherein said waveguide is a ribbon waveguideand wherein said polymer waveguide is a plurality of discrete waveguidefingers formed from said ribbon waveguide, each discrete waveguidefinger being deposed in one of said grooves.
 18. The subassembly ofclaim 17, wherein said ribbon waveguide is notched at one end, saidnotch corresponding to at least a portion of the perimeter of theinterposer such that the interposer is held precisely in positionrelative to said discrete waveguide fingers of said ribbon waveguide.19. The subassembly of claim 13, wherein said waveguide is notched atone end, said notch corresponding to at least a portion of the perimeterof the interposer such that the interposer is held precisely in positionrelative to said waveguide.
 20. The subassembly of claim 13, whereinsaid sides and bottom of said groove are coated with a reflectivematerial, and wherein said optical conduit comprises polymer corematerial disposed in said groove.